[bts89]

> cash and cash equivalents, such as checks or gift cards that can be exchanged for cash, must never be accepted

The way it’s worded sounds like it can be a gift card, just not one that could be exchanged directly for cash.

[harrisi]

The IRS generally considers gift cards as cash equivalent. I'm not a {tax,law,finance,*} professional, but I believe the USPS agrees. I do agree that the text you quoted is a bit ambiguous by itself. "De minimis fringe benefit" is a relevant thing to look up for the curious.

(PDF) https://www.irs.gov/pub/irs-tege/p_4090_fed_0305_text.pdf